hell… padding is even required in FIA cages where helmets are being worn.
I personally would never even think of putting a bolt in cage in any car. The STi has a pretty complex ring construction that already functions as a type of rollcage in crashes, I am not sure if this cage would help or hurt you in an accident, but that uncertainty is what scares me away.
Most “harnesses” unless they are certified and PROPERLY installed/mounted, are not meant for anything more than holding you in place during an autocross.
I would think about running the 5-point system with the anti sub belt as well.